A Triangle and its Circumcircle
This applet demonstrates the construction of the circumcircle of a given triangle ABC. The perpendicular bisectors of the sides of triangle ABC intersect at a point F. The distance from F to each of the points A, B, and C is the same, say r. Therefore, the circle with center F and radius r passes through all three points A, B and C.
In the applet, you may click and drag the points A, B, and C; the corresponding triangle and its circumcircle will be drawn.
Bradley and Stemkoski, Created with GeoGebra